A heat treatment roller hearth furnace is a roller conveyor system for heat treating parts on roller conveyors. For example, heat treatment roller hearth furnaces may be used to harden metallic industrial parts by heating the parts at elevated temperatures. A heat treatment roller hearth furnace may include a plurality of rollers which convey the parts, usually at a slow speed, through one or more temperature zones defined by one or more heating chambers of the furnace. The parts may be placed directly on the rollers or in carrier trays, and the treated parts may be collected at a discharge end of the furnace. The drive systems of roller hearth furnaces may include a motor to drive one or more drive sections, with each drive section driving the rotation of a group of rollers. Current drive section designs for roller hearth furnaces may include a single motor-driven gear reducer which drives the rotation of a chain loop which may, in turn, drive the rotation of the rollers.
With the increasing implementation of automated systems in industrial processes, many heat treatment roller hearth furnaces may be incorporated into an automation cell which may include one or more additional processing stations. For example, the automation cell may include a heat treatment station in which the parts are heated in a roller hearth furnace, a quenching station in which the parts are cooled, and a temper station in which the parts are conveyed through a lower temperature temper furnace to relieve stresses in the parts caused by the heating and/or quenching steps. At each station, the treated parts may be collected by one or more robots and carried to a subsequent station in the automation cell. Each of the stations and the robots of the automation cell may be controlled by a programmable logic controller (PLC). If a piece of equipment breaks at a downstream station or at a robot downstream of the roller hearth furnace station, the PLC may prevent the discharge of treated parts from the roller hearth furnace until the downstream equipment is fixed. Rather than stopping the rotation of the rollers of the roller hearth furnace, the PLC may initiate the drive sections to oscillate the rollers back and forth between the forward and reverse directions in order to prevent heat-induced sagging of the rollers or other heat-induced roller distortion which could occur if the rollers are left idle inside of the high temperature environment of the furnace.
The use of roller hearth furnaces in automation cells has spawned the need for driving the rollers in oscillation, as the conveyance of parts through roller hearth furnaces in automation cells is dependent on the flow of parts through other downstream stations. However, roller oscillation may require the gear reducer to drive or pull through the “slack” side (or loose side) of the chain loop when shifting from the forward to reverse directions. Depending on the degree of chain slack, the chain loop may turn for a variable amount of time before the chain loop is tight and begins driving the rotation of the rollers in the reverse direction. The variable amount of chain slack may cause the system to lose accurate tracking of part positions in the furnace, and this may cause part collisions or even furnace crashes and system downtime during periods of oscillation.
Accordingly, there is a need for improved drive systems for roller oscillation in roller hearth furnaces which reduce or prevent part mis-positioning during oscillation events. U.S. Pat. No. 5,088,990 describes the use of idler gears which mesh with external teeth of secondary drive loops to maintain tensioning of the drive loops and allow uniform reversal of the conveyor rolls as the driving direction reverses. While effective, improved systems for driving the rollers of roller hearth furnaces in oscillation are still wanting. The present disclosure addresses these problems.